Complete beginner question, merging vertices
To merge vertices to corresponding vertices on another loop..

Select the sections of the loops you want to merge, (with an equal vertex count) 
 and use CtrlE > Bridge Edge Loops, with the 'Merge' option checked in the Tool region, or Options panel in 2.8.



as Ray Mairlot has commented, you may intend to merge vertices at opposite ends of existing edges.. in which case you would AltCtrl select the edge ring, followed by X > Edge Collapse.